Determinations of the transient thermal lensing effect in metal cluster Polymer {WS4Cu4I2(bpe)3}n solution by the use of the Z-scan

The nonlinear optical (NLO) properties of a novel cluster Polymer {WS₄Cu₄I₂(bpe)₃}_n solution are studied by using Z-scan technique with laser pulses of 4.5 ns pulse-width at a wavelength of 532 nm. The results show that the cluster solution possesses strong nonlinear absorption and refraction. Nonlinear refraction of the cluster is composed of third-order nonlinear refraction and transient thermal effect. The thermal effect is mainly due to the strong nonlinear absorption. Numerical simulations obtained by solving simultaneously photo-acoustic and electromagnetic wave equations, agrees basically with experimental results.     

Growth and spectral characterization of β-Ga2O3 single crystals

Beta gallium oxide (β-Ga₂O₃) single crystals were grown by the floating zone technique. The absorption spectra and the luminescence of the crystals were measured. The absorption spectra showed an intrinsic short cutoff edge around 260 nm with two shoulders at 270 and 300 nm. Not only the characteristic UV (395 nm), blue (471 nm) and green (559 nm) lights, but also the red (692 nm) light can be seen in the emission spectra. The deep UV light was attributed to the existing of quantum wells above the valence band and the red light was owed to the electron-hole recombination via the vicinity donors and acceptors.     

Diode-pumped Nd:BaY2F8 picosecond laser

Picosecond pulse generation at 1050 nm by a diode-pumped Nd:BaY₂F₈ (Nd:BaYF) active crystal has been investigated under low-power pumping (440 mW absorbed). The sample was oriented along the optical axes and cut at Brewster angle for y-axis laser polarization to minimize birefringence effects. Stable passive mode-locking operation has been achieved by inverse saturable absorption with 80-mW output power and 3.7-ps pulses.     

β-BaB2O4 deep UV monolithic walk-off compensating tandem

The generation of watt-level cw narrow-linewidth sources at specific deep-UV wavelengths corresponding to atomic cooling transitions usually employs external cavity-enhanced second-harmonic generation (SHG) of moderate-power visible lasers in birefringent materials. Among the oxo-borate materials, barium borate (β-BaB₂O₄ or BBO) combines the highest UV band edge and largest nonlinearity but suffers from large walk-off angles that limits the nonlinear interaction length. Alternative quasi-phase-matched (QPM) ferroelectrics are hardly suited for cavity-enhanced operation due to their much larger UV absorption and associated photo-refractive and thermal lensing effects, in addition to the difficult fabrication of fine-pitch domain gratings for short UV coherence lengths. In this work, we investigate an alternative approach to cw deep-UV generation by employing the low-loss BBO in a monolithic walk-off compensating structure [J.-J. Zondy, Ch. Bonnin, D. Lupinski, J. Opt. Soc. Am. B 20 (2003) 1675] to simultaneously enhance the effective nonlinear coefficient while minimizing the UV beam ellipticity under tight focusing. As a preliminary step to cavity-enhanced operation, and in order to apprehend the design difficulties stemming from the extremely low acceptance angle of BBO, we investigate and analyze the single-pass performance of a L_c = 8 mm monolithic walk-off compensating structure made of 2 optically-contacted BBO plates cut for type-I critically phase-matched SHG of a cw λ = 570.4 nm dye laser. As compared with a bulk crystal of identical length, a sharp UV efficiency enhancement factor of 1.65 has been evidenced with the tandem structure, but at ∼−1 nm from the targeted fundamental wavelength, highlighting the sensitivity of this technique when applied to a highly birefringent material such as BBO. Solutions to angle cut residual errors are identified so as to match accurately more complex periodic-tandem structure performance to any target UV wavelength, opening the prospect for high-power, good beam quality deep-UV cw laser sources for atom cooling and trapping.     

Third-order optical nonlinearities of lead-free (Na1−xKx)0.5Bi0.5TiO3 thin films

(Na_1−xK_x)_0.5Bi_0.5TiO₃ (NKBT) (x = 0.1, 0.2, and 0.3) thin films with good surface morphology and rhombohedral perovskite structure were fabricated on quartz substrates by a sol-gel process. The fundamental optical constants (the band gaps, linear refractive indices and absorption coefficients) of the films were obtained through optical transmittance measurements. The nonlinear optical properties were investigated by Z-scan technique performed at 532 nm with a picosecond laser. A two-photon absorption effect closely related with potassium-doping content was found in thin films, and the nonlinear refractive index n₂ increases evidently with potassium-doping. The real part of the third-order nonlinear susceptibility χ⁽³⁾ is much larger than its imaginary part, indicating that the third-order optical nonlinear response of the NKBT films is dominated by the optical nonlinear refractive behavior. These results show that NKBT thin films have potential applications in nonlinear optics.     

A new Yb-doped oxyorthosilicate laser crystal: Yb:Gd2SiO5

A new Yb-doped oxyorthosilicate laser crystal, Yb:Gd₂SiO₅ (Yb:GSO), has been grown by the Czochralski (Cz) method. The crystal structure was determined by means of X-ray diffraction analysis. Room temperature absorption and fluorescence spectra of Yb³⁺ ions in GSO crystal were measured. Then, spectroscopic parameters of Yb:GSO were calculated and compared with those of another Yb-doped oxyorthosilicate crystal Yb:YSO. Results indicated that Yb:GSO crystal seemed to be a very promising laser gain media in generating ultra-pulses and tunable solid state laser applications. As expected, the output power of 2.72 W at 1089 nm was achieved in Yb:GSO crystal with absorbed power of only 4.22 W at 976 nm, corresponding to the slope efficiency of 71.2% through the preliminary laser experiment.     

Investigation of Nd:YVO4/YVO4 composite crystal and its laser performance pumped by a fiber coupled diode laser

Thermal effect control is critical to scale the output power of diode end-pumping solid lasers to several watts up and beyond. Diffusion bonding crystal has been demonstrated to be an effective method to relieve the thermal lens for the end-pumping laser crystal. The temperature distribution and thermal lens in Nd:YVO₄/YVO₄ composite crystal was numerically analyzed and compared with that of Nd:YVO4 crystal in this paper. The end-pumping Nd:YVO₄/YVO₄ composite crystal laser was set up and tested with z cavity. The maximum output power of 9.87 W at 1064 nm and 6.14 W at 532 nm were obtained at the pumping power of 16.5 W. The highest optical-optical conversion efficiencies were up to 60% at 1064 nm and 40% at 532 nm, respectively.     

Efficient blue upconversion emission due to confined radiative energy transfer in Tm-Nd co-doped Ta2O5 waveguides under infrared-laser excitation

Intense blue upconversion emission at 480 nm has been obtained at room temperature in Tm³⁺-Nd³⁺ co-doped Ta₂O₅ channel waveguides fabricated on a Si substrate, when the sample is excited with an infrared laser at 793 nm. The upconversion mechanism is based on the radiative relaxation of the Nd³⁺ ions (⁴F_3/2 → ⁴I_11/2) at about 1064 nm followed by the absorption of the emitted photons by Tm³⁺ ions in the ³H₄ excited state. A coefficient of energy transfer rate as high as 3 × 10⁻¹⁶ cm³/s has been deduced using a rate equation analysis, which is the highest reported for Tm-Nd co-doped systems. The confinement of the 1064 nm emitted radiation in the waveguide structure is the main reason of the high energy transfer probability between Nd³⁺ and Tm³⁺ ions.     

Growth and optical properties of a new self-Q-switched laser crystal: Cr:Yb3Al5O12

Cr⁴⁺:YbAG (Cr⁴⁺:Yb₃Al₅O₁₂) crystal with a size up to Φ24 mm×30 mm was grown by the Czochralski method. In the absorption spectrum, there are two absorption bands at 939 and 969 nm, respectively, which are suitable for InGaAs diode laser pumping, and there is an absorption band at 1030 nm, which is suitable for passive Q-switched laser output at 1.03 μm. A broad emission spectrum from 970 to 1100 nm was exhibited from 940 nm wavelength pumping. This crystal is promising as a self-Q-switched laser crystal used for compact, efficient, highly stable, passive self-Q-switched thin chip solid-state lasers.     

Ultra-broadband optical parametric amplification in KBe2BO3F2 crystal

It has been demonstrated for the first time that ultra-broadband phase-matching coupled with group-velocity matching is possible in a type-I non-collinear optical parametric amplifier in a recently grown KBe₂BO₃F₂ (KBBF) crystal, pumped by Ti:sapphire second harmonic 395 nm radiation and seeded by white-light continuum.     

Third order optical nonlinearities in ZnFe2O4 nanocrystals

Using Z-scan method with picosecond laser at 532 nm, the third order optical nonlinearities of ZnFe₂O₄ were investigated. The nonlinear refractive index is positive for all ZnFe₂O₄ samples and decrease with the nanocrystal size. The nonlinear absorption dominated by saturable absorption for 19 nm and 11 nm ZnFe₂O₄ but by two photon absorption for 5 nm ZnFe₂O₄ organosol. Origin of the optical nonlinearities and the size effect has been discussed.     

Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers

The objective of this study was to identify a material suitable to absorb radiation at the wavelength of neodymium-doped Yttrium Aluminum Garnet (Y₃Al₅O₁₂:YAG), 1064 nm. M-(M= Sm³⁺, Co²⁺, Co³⁺, Cr³⁺, and Cr⁴⁺) doped highly transparent YAG ceramics were fabricated, and their absorption spectra were measured. Unlike Co²⁺ and Cr³⁺-doped ceramic samples, Co³⁺ and Cr⁴⁺ and Sm³⁺-doped:YAG ceramics were found to have significant absorption at 1064 nm. However, the Sm³⁺-doped YAG clearly emerged as the best candidate because it is also transparent at 808 nm, the pumping wavelength laser diode (LD), and also at most absorption bands used for flash-lamp pumping.     

Diode-pumped passively Q-switched Nd:GdVO4 laser at 1342 nm with V:YAG saturable absorber

We have demonstrated an efficient diode-pumped passively Q-switched Nd:GdVO₄ laser working at 1342 nm by using an uncoated V³⁺:YAG crystal as the saturable absorber, in which both a-cut and c-cut Nd:GdVO₄ crystals are employed. At the maximum absorbed pump power of 9.45 W, the maximum average output power can reach 519 mW and 441 mW corresponding to the output coupler with different transmission of 3% and 10% by using an a-cut Nd:GdVO₄ crystal at 1342 nm, while the shortest pulse duration could be as low as 21.7 ns and 22.3 ns with the repetition rate of 48.41 kHz and 53.25 kHz by using a c-cut Nd:GdVO₄ crystal, corresponding to the output coupler with different transmission of 3% and 10% at 1342 nm, and the single Q-switched pulse energy are 6.67 uJ and 7.06 uJ, the pulse peak power are 307 W and 316 W, respectively. The experimental results show that c-cut Nd:GdVO₄ laser can generate shorter pulse with higher peak power in comparison with a-cut one.     

Highly efficient frequency doubling of high-energy Nd:YAG laser using a CsB3O5 crystal

We have demonstrated efficient frequency doubling of high-energy fundamental Nd:YAG laser pulse energy of the multi-joule (J) level at a high repetition rate using high optical-quality top-seeded solution growth CsB₃O₅(TSSG-CBO) crystal for the first time. Second-harmonic (532 nm) generation (SHG) output energy of 1.2 J at 10 Hz is obtained with a conversion efficiency of 60%. This result has been obtained at the multi-J level by the growth of high optical-quality TSSG-CBO crystal with the large effective nonlinear coefficient and high damage threshold. These results indicate that TSSG-CBO is a good candidate material for high-energy SHG of Nd-doped lasers at the several J level or more with high repetition rate.     

Synthesis and nonlinear optical characterization of SnO2 quantum dots

The present paper demonstrates the preparation and characterization of SnO₂ semiconductor quantum dots. Extremely small ∼1.1 and ∼1.4 nm SnO₂ samples were prepared by microwave assisted technique with a frequency of 2450 MHz. Based on XRD analysis, the phase, crystal structure and purity of the SnO₂ samples are determined. UV-vis measurements showed that, for the both size of SnO₂ samples, excitonic peaks are obtained at ∼238 and ∼245 nm corresponding to ∼1.1 nm (sample 1) and ∼1.4 nm (sample 2) sizes, respectively. STM analysis showed that, the quantum dots are spherical shaped and highly monodispersed. At first, the linear absorption coefficients for two different sizes of SnO₂ quantum dots were measured by employing a CW He-Ne laser at 632.8 nm and were obtained about 1.385 and 4.175 cm⁻¹, respectively. Furthermore, the nonlinear refractive index, n₂, and nonlinear absorption coefficient, β, were measured using close and open aperture Z-scan respectively using the same laser. As quantum dots have strong absorption coefficient to obtain purely effective n₂, we divided the closed aperture transmittance by the corresponding open aperture in the same incident beam intensity. The nonlinear refraction indices of these quantum dots were measured in order of 10⁻⁷ (cm²/W) with negative sign and the nonlinear absorption coefficients were obtained for both in order of 10⁻³ (cm/W) with positive sign.     

Spectroscopic investigation of Tm:TeO2-WO3 glass

We studied the spectroscopic characteristics of telluride glass with the host composition (0.85)TeO₂-(0.15)WO₃, containing 0.25 and 1.0 mol% thulium oxide (Tm₂O₃). By analyzing the absorption spectra with the Judd-Ofelt theory, the average radiative lifetimes of 305±7.5 μs and 1.95±0.02 ms were determined for the ³F₄ and ³H₄ levels, respectively. Measured fluorescence lifetime of the ³F₄ level decreased from 218 to 51 μs for the 0.25 and 1.0 mol% Tm₂O₃ doped samples, respectively, indicating the effect of boosted non-radiative decay at higher doping concentrations. A similar trend was observed for the ³H₄ level, where the fluorescence lifetime decreased from 1.86 ms to 350 μs at these concentrations. The quenching of the 1460 nm (³F₄→³H₄) emission in favor of the 1800 nm (³H₄→³H₆) emission due to cross relaxation was further evident in the fluorescence spectra of the samples. The calculated stimulated emission cross sections (3.73±0.1×10⁻²¹ cm² at 1460 nm and 6.57±0.07×10⁻²¹ cm² at 1808 nm) reveal the potential importance of the Tm³⁺:(0.85)TeO₂-(0.15)WO₃ glass for applications in fiber-optic amplifiers and fiber lasers.     

Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.     

Vacuum ultraviolet luminescence from a micro-pulling-down method grown Nd:(La0.9,Ba0.1)F2.9

Vacuum ultraviolet (VUV) luminescence from a Nd³⁺:(La_1−x,Ba_x)F_3−x (x=0.1) and a Nd³⁺:LaF₃ single crystal grown by the micro-pulling-down method modified for fluoride crystal growth is discussed. Emission resulting from excitation with 157 nm pulses of a F₂ laser and by 290 nm femtosecond pulses of a Ti:sapphire laser show that the luminescence spectral and temporal characteristics are similar for both excitation cases and that they have good prospects as a VUV laser material.     

Emission characteristics, crystalline phase and composition of vapor-transport-equilibrated Er:LiNbO3 crystal codoped with 6 mol% MgO

Polarized downconversion, 980-nm-upconversion and near-infrared emission characteristics of vapor-transport-equilibrated (VTEed) bulk Er (0.4 mol%)/MgO (6 mol%)-codoped LiNbO₃ crystals were investigated. The downconversion and upconversion visible emissions display similar VTE effects including the drop of emission intensity and the weakening of polarization dependence. At 0.98 and 1.5 μm regions, the VTE has a weak effect on the emission intensity, but a strong effect on the spectral shape. The crystalline phases in these bulk Er/Mg-codoped VTE crystals are determined by comparing their infrared emission characteristics with those of pure ErNbO₄ powder and locally Er-doped MgO (4.5 mol%):LiNbO₃ crystal. The results show that the Er³⁺ ions present in these bulk Er/Mg-codoped VTE crystals as a mixture of Er:LiNbO₃ and ErNbO₄ phases. The percentages of the ErNbO₄ phase contained in these VTE crystals were evaluated from the 1531 and 1536 nm characteristic absorption areas. The contents of constituent elements were determined by chemical analysis.     

Nonlinear optical properties in SrTiO3 thin films by pulsed laser deposition

Well-crystallized 250 nm-thick SrTiO₃ thin films on fused-quartz substrate were prepared by pulsed laser deposition. The band-gap of SrTiO₃ thin film by transmittance spectra is equal to 3.50 eV, larger than 3.22 eV for the bulk crystal. The nonlinear optical properties of the films were examined with picosecond pulses at 1.064 μm excitation. A large two-photon absorption (TPA) with absorption coefficient of 87.7 cm/GW was obtained, larger than 51.7 cm/GW for BaTiO₃ thin films. The nonlinear refractive index n₂ is equal to 5.7×10⁻¹⁰ esu with a negative sign, larger than 0.267×10⁻¹¹ esu for bulk SrTiO₃. The large TPA is attributed to intermediate energy levels introduced by the grain boundaries, and the optical limiting behaviors stemming from both TPA and negative nonlinear refraction were also discussed.